Nonmagnetic CVs

If the field cannot effectively channel the flow near the white dwarf, an equatorial BL forms in which the disk material is braked from the Keplerian angular velocity Qk to the stellar value Q*. If the BL is optically thin as expected for dwarf novae in quiescence, the accreted matter is heated roughly to (or to a fraction of) the virial temperature kTvir = ^muGM( 1 -o2)/3R ~ 49 (1 -o2)(M/MQ)19 keV, where ^ is the mean molecular weight (0.617 for solar composition), mu is the unit mass, o = Q*/Qk(R), and the mass radius relation for white dwarfs with Teff ~ 20000 K has been approximated by R ~ 5.8 x 108 (M/M0)-0-9 cm. Shocks, evenmultiple shocks, may occur in the flow. While the structure of the BL is not yet fully understood, it does represent some kind of cooling flow with temperatures ranging from Tvir (or a fraction thereof) down to the equatorial surface temperature of the white dwarf. With increasing M, the BL becomes optically thick and its luminosity and blackbody temperature are given by where the emitting area is a narrow equatorial belt of width 2H and fractional area f = 2nR x 2H/4kR2 = H/R, which is of the order of 0.01. Internal absorption is quite likely to be important as is absorption and scattering in the wind that emanates from a luminous CV disk. If the inclination is low, spectral flux will be scattered out of the line of sight and if the inclination is high and the BL is hidden from view, flux will be scattered into the line of sight [54]. The sketch in the left-hand diagram of Fig. 12.1 gives a rough idea of the geometry [65].

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